chmod in mix task fix

During ``mix weber testWeberApp`` execution:
```
** (FunctionClauseError) no function clause matching in :file.change_mode/2
    file.erl:1160: :file.change_mode("/home/mayoi/dev/tmp/weber/testWeberApp/start.sh", "+x")
    /opt/elixir/lib/mix/lib/mix/cli.ex:44: Mix.CLI.run_task/2
    /opt/elixir/lib/elixir/lib/code.ex:290: Code.require_file/2
    /opt/elixir/lib/elixir/lib/kernel/cli.ex:308: Kernel.CLI.process_command/2
    /opt/elixir/lib/elixir/lib/enum.ex:678: Enum."-map/2-lc$^0/1-0-"/2
```

According to Erlang documentation, ``file:change_mode`` accepts only integers as mode.